Spoken dialog systems have evolved to handle a broad range of queries covering a growing number of topics. Rather than being confined to the duration of a phone call, such as used to be the case with telephony-based systems, newer generations of virtual agents are continuously online and can be summoned by simply speaking a wake-word. Speech-based assistants can be implemented by various types of devices, including smartphones; media players; home automation systems; electronic peripherals; various types of machinery and equipment; computers such as tablet, laptop, and desktop computers; and so forth.